Hi,
I've been trying to get this to work and it does for awhile. When pulled low the current passes from the solar panel array through the P channel MOSFET to the boost/buck battery charger just fine, but when the control signal from my BMS floats high at full charge the MOSFET gets really hot really fast and fails shorted. please take a look at this and tell me what's wrong. MOSFET info: https://www.infineon.com/dgdl/Infin...fileId=db3a30432f69f146012f783b3b5a2e3f&ack=t

 

What is the condition for a P MOSFET to be OFF?
You're not meeting it or even coming close.
Do you know the difference between the source and the drain?
The diagram doesn't show it.

 

What is the condition for a P MOSFET to be OFF?
You're not meeting it or even coming close.
Do you know the difference between the source and the drain?
The diagram doesn't show it.

sorry about the diagram, i'll work on it. I assumed it to be OFF when not grounded, pulled high, by difference between source and drain do mean potential?

 

Title: Understanding Basic Electronics, 1st Ed.
Publisher: The American Radio Relay League
ISBN: 0-87259-398-3

 

Title: Understanding Basic Electronics, 1st Ed.
Publisher: The American Radio Relay League
ISBN: 0-87259-398-3

Thanks BobaMosfet, can you provide a chapter reference? I was hoping to get a quicker solution than buying a book.

 

sorry about the diagram, i'll work on it. I assumed it to be OFF when not grounded, pulled high, by difference between source and drain do mean potential?

I lOOked again at my drawing and now I see what you mean, ill redraw and edit my OP

 

What is the condition for a P MOSFET to be OFF?
You're not meeting it or even coming close.
Do you know the difference between the source and the drain?
The diagram doesn't show it.

corrected schematic is now up, mind taking another look and see what i'm missing

 

An enhancement mode device has a threshold voltage which is kind of a boundary between being on or off. It is usually measured by the magnitude and direction of the gate to source voltage. Power dissipation can be very high when a device is in between fully on and fully off. For a PMOS device you want the source connected to the higher voltage. As the gate voltage drops below the source voltage the device starts to turn on. At some point you reach Vgs(th), the gate to source threshold voltage, which is negative, with respect to the source. For example, if the source is at +20VDC and the gate is at +16VDC it is actually at -4V with respect to the source. This is the danger zone for power dissipation and you want to quickly get the gate voltage to -10V or -20V with respect to the source. Now the device is fully on and everything is charry. When the battery is charged you do not want the gate to float back to the -4V threshold region. That is the road to perdition. You want that sucker to snap back like a rifle shot to 0V with respect to the source.

Got it?

 

An enhancement mode device has a threshold voltage which is kind of a boundary between being on or off. It is usually measured by the magnitude and direction of the gate to source voltage. Power dissipation can be very high when a device is in between fully on and fully off. For a PMOS device you want the source connected to the higher voltage. As the gate voltage drops below the source voltage the device starts to turn on. At some point you reach Vgs(th), the gate to source threshold voltage, which is negative, with respect to the source. For example, if the source is at +20VDC and the gate is at +16VDC it is actually at -4V with respect to the source. This is the danger zone for power dissipation and you want to quickly get the gate voltage to -10V or -20V with respect to the source. Now the device is fully on and everything is charry. When the battery is charged you do not want the gate to float back to the -4V threshold region. That is the road to perdition. You want that sucker to snap back like a rifle shot to 0V with respect to the source.

Got it?

I got it. Do you see anyway to do this with what I’ve got?

 

I got it. Do you see anyway to do this with what I’ve got?

Not really. For additional insight on battery technology and charging techniques, I recommend you spend some time at Battery University.

https://batteryuniversity.com/

If you spend some time there you will pickup a good deal of useful information. At your level of knowledge, I would consider buying an off the shelf situation for your immediate needs. Once you have learned about your particular battery chemistry, you can go about experimenting with alternatives. You will save money and aggravation in the long run with this approach.

 

Not really. For additional insight on battery technology and charging techniques, I recommend you spend some time at Battery University.

https://batteryuniversity.com/

If you spend some time there you will pickup a good deal of useful information. At your level of knowledge, I would consider buying an off the shelf situation for your immediate needs. Once you have learned about your particular battery chemistry, you can go about experimenting with alternatives. You will save money and aggravation in the long run with this approach.

Thanks, I built a similar circuit that works on the discharge side pretty well. I guess I’ll go back to the automotive type relays on the charge side. One of the of the reasons I went down the diy path was to learn something about electronics and this forum has been very helpful. I like the community and the level of snark is just about right for me. Thanks again

 

Doesn't the REDARC charger have a shutoff feature. I mean, it should be able to withstand any voltage at its input and not pass it on to the batteries. Is this a mistaken assumption on my part?

If you do want to continue on the path, I would investigate the LM339(quad) or LM393(dual) comparator to control the gate voltage. The inputs can look at the difference between battery voltage and some fraction of the solar panel voltage. The output is open collector, so a resistor from the source to the gate to the comparator output might do the job. The size of the resistor will determine how fast the gate-source voltage will equalize when the device turns off.

http://www.ti.com/lit/ds/symlink/lm393-n.pdf?ts=1588790084469

 

I’ll look into the Redarc specs and see. I wanted to be able to set a slightly lower level of charge on this first level of redundancy. The Redarc is a pretty nice gadget in that it combines truck alternator output with solar to provide a full 25 amps (or 45) while driving. Output I measure is at 14.55V, so maybe I could just connect straight up. I’m using an OrionBMSjr and using the charge and discharge enable outputs to drive a 4x555 pulse generator which in turn control large Tyco latching relays. I got a lot of help from Crutschow on that project (see: “Just can’t leave well enough alone...” thread

 

Redarc shows in their install manual, the device connected directly to the LiFePO4 through an in-line fuse. I'll give a try!